Fosi Audio SK01 Headphone Amplifier Equalizer Preamp with Bass Midrange Treble Tone Control and Loudness Compensation Portable Class A Analog Amp for Electric Guitar Instrument with 3.5mm 6.35mm Jacks

Fosi SK-01 is an extremely well-constructed audio "enhancer" that has become almost indespensible in my different audio systems. Solid metal with some heft, the knobs fit tightly and move smoothly. A very slight adjustment is noticed almost immediately in the soundscape with the tone controls, the volume knob is slightly larger and provides easy access. I like the sound quality that the unit outputs and have used the SK-01 with various class D amps with only volume control - this unit is perfect for adding EQ adjustments. The tone defeat will bypass any EQ and play neutral, another feature that is really appreciated for my usage. The loudness button will bump certain frequencies and is a nice feature when playing at lower volumes. I have no complaints of this product and commend Fosi Audio for designing and bringing this device to the audio market. The 2500MaH battery will last me for several days if needed and the unit can run constant on USB 5V power. Also can work very nicely with phone/tablet/sound card as source and a usb "C" adapter with 3.5mm out will work perfectly as input source to the SK-01. There is a 3.5 headphone out, and also a 6.25" - I haven't used the SK-01 much with headphones but it has that option covered.. Great all-around device, I purchased another unit after 3 months of heavy use and I will probably always have one of these units on hand to modify an analog signal.

The best sounding results were found when streaming movies or from .FLAC audio files when pairing this unit with a SK02 USB DAC using its Aux out. I found that using the SK02's AUX/OPT 3.5mm connect jack (which also doubles as an optical ouput jack as well) gave a higher output level than the headphone jack. Using a 12" long 3.5mm male TRS to 3.5mm male TRS from the Aux Out of the SK02 into the Line IN of the SK01 provided the best undistored output when connecting a 3.5mm to RCA splitter from the SK01 Line out to a 75WPC Amp. This preamp's mid range control provided a lot better tone than the K5 pro that did not have this mid adjustability. An added feature is its internal battery that is charged from the USB-C connection.

All I wanted was to bring out the subtle sounds in the accompanying music mix without mushing up the vocalist's track. I use Moondrop Blessing in-ear-monitors (earbuds) ...https://www.amazon.com/gp/product/B0C1ZBTD2Vand a conventional CD player (KLIM Discover)...https://www.amazon.com/dp/B0BRXH9L84with an upgrade cable (GUCraftmans cable with two pin connectors to match the Moondrop Blessing earbuds)...https://www.amazon.com/gp/product/B08N6211FFI didn't want a DAC (Digital-to-Analog Converter) or a combo DAC & Amplifier because the DAC/Amplifiers require RCA inputs or OPTICAL inputs. My CD player has only one type of output: 3.5mm (for earphones).Instead, I just wanted a headphone amplifier that would sweeten the sound of my CDs. I wanted to use my CD's player's headphone output jack (standard 3.2mm headphone jack) to connect the CD player to the Headphone Amplifier's 3.2mm input. I would then plug in my earphones to the output headphone jack on the amplifier.This amplifier is special because you can adjust the sound by manually altering the tones for bass, mids, and treble. Or, by pressing a simple button, you can bypass all tone adjustments and just let the amplifier do all the work of adjusting the volume of the track. Even without the tonal adjustments disabled, music still sounds better than without the amplifier.There are two more adjustments that can be enabled or left alone. The manufacturer describes them as follows: "Diverse Capabilities: Instantly restore original sound with the one-click tone-defeating button (I explain that one above); Immerse yourself in music during the quiet hours using the loudness function; Tailor the gain level to accommodate headphones with various impedances through the gain level switch."The loudness function is a stumper for me. I hear no difference when I press the button. I am NOT very responsible when it comes to the volume streaming into my head through the earbuds. I like it loud. I imagine the loudness function is used to keep all sounds under a certain decibel. Or, enabling the function might make the quieter sounds louder, relative to surrounding sounds. It might a "limiter" of some sort.Your own headphones or speakers or in-ear-monitors should have their respective impedance ranges documented. But whether I set the amplifier gain control to 0 or to -10, I don't know which I should choose. Depending on the content on a particular CD, I choose either setting to determine which setting sounds best.The exact title of this amplifier is "Fosi Audio SK01 Headphone Amplifier Equalizer Preamp with Bass Midrange Treble Tone Control and Loudness Compensation Portable Class A Analog Amp for Electric Guitar Instrument with 3.5mm 6.35mm Jacks," but don't let the title throw you. Yes, an electric guitar player can use this amplifier to monitor his guitar-playing, but it can be used for just about anything that you would want to make louder and sweeter...like your collection of CDs!The Fosi Audio SK01: https://www.amazon.com/gp/product/B0CF1WL1V2

This things is politically irreverant. For one, it apparently doesn't measure well in an era where other headphone amplifiers achieve technical perfection at really low costs. Worse yet, it has three(!) tone controls and a loudness EQ too. Heresy, heresy! We're supposed to just buy poor performing cables or overly dull or bright headphones to compensate for the general tone in our favorite recordings. It also has no DAC or balanced headphones support.I'm using this with a 70s Akai tape recorder. Just imagine how bad the headphone out is on that! Hooked the deck up to a 90s Onkyo reciever, and vast improvement but can still hear hiss and audible distortion through headphones. As limited as thsAnalog tape audio is an excellent rationale for tone controls by the way. Recordings may have been made on misaligned or dirty equipment, dubbed onto borderline defective tape stock, or just recorded and mastered with a casual and in some cases cavalier disregard for tonal balance. It's an aesthetic, yes, and if everything is already aestheticized, why not dial the aesthetic in to one's own preference thereof?

Solid build, dials feel good.The only pre-amp that I found that included mid tone adjustment.Good battery life.Loudness control is a nice option -- not too common with preamps.Size is good, fits in your hand. Can be mobile, not the smallest profile, but can take it with you.Descent power output, works well with my Sennheiser HD 600 and Hifiman Sundara headphones.Good price to feature ratio.Overall very pleased with my purchase of this amp.